This invention relates to the writing of Bragg reflection gratings in optical waveguides by exercise of the photorefractive effect in materials, such as germania doped silica. An early method of writing such gratings is described in U.S. Pat. No. 4,474,427, this method involving directing intense ultra-violet light into one end of a length of optical waveguide having a reflector at the far end so that a standing wave is set up within the waveguide. Subsequently, a lateral method of writing such a grating was disclosed in U.S. Pat. No. 4,725,110 in which a beam-splitter is used to divide a beam of ultra-violet light into two beams which are reflected in two mirrors to form and interference fringe pattern in the waveguide. This writing method is sometimes known as the holographic method. A further alternative method, sometimes known as the grating method, involves creating an equivalent fringe pattern close behind a diffraction grating illuminated with normally incident ultra-violet light, this method being described for instance in U.S. Pat. No. 5,351,321. Typically a phase grating is employed for this purpose, and the depth of the grating elements chosen for suppression of the zero order diffraction. The waveguide is located close behind the diffraction grating because it is here that the required fringe pattern, which is generated by interference between the +1 and -1 diffraction orders, is least disrupted by power diffracted into higher order modes.
An exception to this close proximity arrangement of diffraction grating and waveguide is described in the paper by J R Armitage entitled, `Fibre Bragg Reflectors written at 262 nm Using a Frequency Quadrupled Diode-Pumped Nd.sup.3+ :YLF Laser`, Electronics Letters, 24th Jun. 1993 Vol. 29, No. 13, pages 1181-3. In this instance the +1 and -1 diffraction orders are arranged to be incident upon a fused silica block. After reflection in the side walls of this block, the two beams emerge from the far end of the block and form an interference fringe pattern in the core of an optical fibre waveguide located behind the block. One advantage of this greater separation between the diffraction grating and the waveguide is that it enables a zero order beam stop to be located on the fused silica block to prevent any zero order light from reaching the fringe pattern generated by the interference between the +1 and -1 diffraction orders and thereby reducing the visibility of that fringe pattern.